Casbin vs. Open Policy Agent (OPA)

Open Policy Agent (OPA) and Casbin are both open-source policy engines, and they are often shortlisted together. OPA is a general-purpose engine: one language, Rego, for decisions across Kubernetes, infrastructure, and services. Casbin concentrates on access control inside applications, with rules stored as data that can be changed at runtime.

Summary

	Casbin	OPA
Scope	Application access control	General-purpose policy for any JSON input
How rules are written	A short model file plus policy rules as rows	Programs in the Rego language
Typical deployment	Library in your process	Sidecar or daemon queried over HTTP; also a Go library and WebAssembly
Native implementations	Go, Java, Node.js, Python, .NET, PHP, Rust, C++, and more	Go
Where rules and data live	Your database, through adapters	Bundles and data documents loaded into OPA's memory
Changing permissions	Add or remove a policy row through the Management API	Change the data or publish a new policy bundle
Who usually edits rules	Application administrators, through a UI or API	Engineers, through code review
Governance	Apache Software Foundation (Incubating)	Cloud Native Computing Foundation

Rego versus model and policy

In OPA you write the decision logic itself in Rego:

package app.authz

default allow := false

allow if {
    some role in data.user_roles[input.user]
    some grant in data.role_grants[role]
    grant.action == input.action
    grant.resource == input.resource
}

Rego is expressive. It can inspect arbitrary nested JSON, which is why OPA is used for Kubernetes admission control and configuration checks as well as for API authorization. It is also a language your team has to learn, test, and review.

In Casbin the logic is one line in a model file and the grants are rows:

[matchers]
m = g(r.sub, p.sub) && r.obj == p.obj && r.act == p.act

p, editor, article, write
g, alice, editor

Most access control requirements (roles, role hierarchies, tenants, resource ownership, path patterns) fit this shape; see Supported models. For logic the built-in operators do not cover, a matcher can call a custom function written in your application's language.

Managing permissions at runtime

Applications usually need an administration screen where someone assigns roles and grants permissions. With Casbin this is a direct API call:

e.AddRoleForUser("bob", "editor")
e.AddPolicy("editor", "comment", "write")

The change is enforced immediately, saved through the adapter to your database, and propagated to other instances by a watcher. The Management API and RBAC API cover listing and querying as well.

With OPA, the equivalent is a data document that your own service maintains and ships to every OPA instance, through bundles or the data API. That works well, and the storage, distribution, and admin API are yours to build.

Deployment and languages

OPA is written in Go. Go programs can embed it; other languages normally query an OPA process over HTTP or run policies compiled to WebAssembly.

Casbin has native implementations for Go, Java, Node.js, Python, .NET, PHP, Rust, C++, and others. A check is an in-process function call with no serialization or network hop. The same model and policy files work unchanged across languages.

Infrastructure policy

For Kubernetes admission control and cloud configuration, OPA with Gatekeeper is the established choice and has a large library of ready-made policies. Casbin can also be used there, with K8s-Gatekeeper and Envoy integrations, but its strength is application authorization.

When to choose which

Choose OPA if you want one policy language across infrastructure and services, your decisions depend on complex structured input, or policies should be authored and versioned as code by engineers.

Choose Casbin if you are adding roles and permissions to an application, permissions must be editable at runtime by administrators, you want rules stored in your existing database, or you want a native library in your language with no extra process.

Try Casbin

Test a model and policy in the online editor, or follow the tutorial for Go, Node.js, Python, or Java.

See also the comparison overview.